1. Field of the Invention
The present invention relates to a rotary stencil printer, and more particularly to the construction of a printing drum thereof.
2. Description of the Prior Art
In order for a rotary stencil printer to quickly start up to operate normally after the start thereof such that high quality prints are available from the first or second print with almost no trial printing, and in view of the matter that, in a rotary stencil printer wherein each printing paper is pressed between a printing drum and a back press roller for each printing, while the printing drum and the back press roller must be retracted from one another at each interval between two successive supplies of printing sheets, either the printing drum or the back press roller each having a substantial mass must inevitably be frequently reciprocated, thereby substantially restricting an increase of printing speed, it has been proposed in Japanese Patent Application 63-28553 (Laid-open Publication 1-204781) by the same assignee as that of the present application to construct a rotary stencil printer such that a principal portion of the printing drum extending between opposite axial ends thereof to support a stencil sheet wrapped therearound is constructed by a flexible perforated sheet, instead of the conventional printing drum entirely made of a rigid cylindrical body, whereby a part of the cylindrical body is bulged radially outwardly by an internal press roller adapted to rotate along the inner surface of the cylindrical body, such that, according to a rotation of the printing drum in the printing operation, each portion of the flexible cylindrical body opposing the back press roller is successively bulged out by the internal press roller so as to apply a stencil printing to a printing sheet pressed between the bulged out portion of the flexible cylindrical body and the back press roller. Further, as an improvement of such a rotary stencil printer, it has also been proposed in Japanese Patent Application 1-47029 (Laid-open Publication 2-225078) by the same assignee as that of the present application to construct a printing drum such that the above-mentioned flexible cylindrical body is provided by a flexible perforated sheet having a rectangular configuration in development, said flexible perforated sheet being mounted around two annular portions forming opposite axial end portions of the printing drum with opposite side edge portions thereof being slidably seated on the outer circumferential surfaces of the two annular portions.
When the above-mentioned flexible perforated sheet is a net-like sheet made of woven or non woven or knitted fibrous materials, and when such a sheet cylindrically wound with opposite end portions thereof seated around the outer circumferential surfaces of the two annular portions is pressed radially outwardly at an axially internal portion thereof by the internal press roller, the flexible perforated sheet is bulged radially outwardly substantially uniformly over the entire axial length or width thereof contacted by the internal press roller such that a stencil print is available to have a uniform quality over the entire region of width. In this respect, when the opposite side edge portions of the flexible perforated sheet having a rectangular configuration in development are freely seated around the outer circumferential surfaces of the annular portions as arranged according to the afore-mentioned improvement, the cylindrical body shows a higher flexibility so that a higher uniformity is available in the radially outward bulging out thereof under the action of the internal press roller.
However, in order that the cylindrical body is more uniformly bulged radially outwardly by the internal press roller over the entire region of the width thereof, and in order that the printing speed is further increased, it is desired that the cylindrical body has a construction which allows for a more light and uniform bulging out thereof in response to the radially outward pressing action by the internal press roller.